While not limited thereto, the present invention is particularly adapted for use as a steel for roller bearing races which typically have a wall thickness of 1 inch or less for bearings having an O.D. of about 5 inches. Such bearing races should have a relatively soft inner matrix, a carburized outer surface having a Rockwell C hardness of least 58 and the ability to retain properties such as hardness at operating temperatures in the range of 800.degree.F to 1000.degree.F. The steel used for the races must meet two requirements. First, it must be capable of being carburized and heat treated to achieve the desired surface hardness. Secondly, it must be capable of being pierced in a seamless tube-forming operation to achieve the desired wall thickness. This latter parameter is measured by the hot twist tests. If the twist characteristics of the alloy are unsatisfactory, the only way in which a bearing race can be produced is by a costly machining operation on solid bar stock or by ring forging from a billet.
In Jatczak U.S. Pat. No. 3,382,064 a steel alloy is described for bearing races which can be readily carburized such that it is adapted for continuous operation at temperatures up to 800.degree.F to 900.degree.F, or for intermittent operation at 1000.degree.F. It contains as essential constituents, carbon, manganese, silicon, chromium, molybdenum, and vanadium. Although the steel described in the aforesaid patent can be carburized to achieve the desired hardness for bearing operation at high temperatures, its twist data characteristics are not altogether satisfactory with the result that hot piercing operations can be achieved only with difficulty.
In McMullan U.S. Pat. No. 2,624,687, another bearing steel is disclosed having a composition similar to that of Pat. No. 3,382,064 but wherein vanadium and aluminum are not employed as alloying elements, and nickel is added. The purpose of the nickel, according to the McMullan patent, is as an austenite forming element which increases carbon or carbide solubility at lower temperatures, offsetting the reverse effect of molybdenum and chromium.
One drawback of the steel disclosed in the aforesaid McMullan U.S. Pat. No. 2,624,687 is that it contains 1 to 3% chromium and preferably 1.5% chromimum. This amount of chromium increases the difficulty encountered in carburizing and tends to cause grain boundary carbides. Another difficulty with the vanadium free steel of the McMullan patent is that it cannot fully utilize the secondary hardening phenomenon resulting from precipitation of alloy carbides on tempering or aging after hardening by quenching from high austenitizing temperatures. The reasons for this are twofold: (1) it cannot be austenitized from temperatures above 1750.degree.F without excessive grain growth and (2) temperatures around 2000.degree.F are required to produce the solution of the carbide forming elements which induce high secondary hardening on tempering.
Some of the disadvantages of the steel of McMullan are eliminated in Jatczak U.S. Pat. No. 3,382,064. For example, the addition of vanadium impedes grain growth and permits the steel to be quenched from higher temperatures (i.e., 2100.degree.F) during hardening; and it contains lower amounts of chromium which contributes to a greater ease of carburization. On the other hand, as was pointed out above, the steel of U.S. Pat. No. 3,382,064 is not readily pierceable in a seamless tube-forming operation as measured by hot twist data. Thus, both of the foregoing prior art steels and the processes set forth in the respective patents have deficiencies in certain respects.